This invention relates to a method of treating the flue dust of industrial processes, such as the flue dust from steel making processes which contain such materials as zinc, lead or cadmium, to segregate and thereby recover valuable constituents.
Flue dust from certain steel making operations such as electric arc furnaces is classified as a hazardous waste material because of the presence of lead and cadmium. Also, it has been reported that electric arc furnace dust xe2x80x9ccontains several thousand nanogrammes of dioxins per kilogramme of dustxe2x80x9d. xe2x80x9cValue from EAF Dustxe2x80x94The ADPL Processxe2x80x9d, by Kevin Holliday, published Apr. 10, 2000. Yet electric arc furnace dust contains valuable components if separation of the various constituents can be made economically. A process for recovering steel mill and foundry dusts containing zinc is disclosed in U.S. Pat. No.3,850,613, which is herein incorporated by reference in its entirety. The flue dust is formed into compacts and then heated to reduce the zinc and lead to a metal, and then to volatilize the metal. The metal vapor is then combined with oxygen to form gas-borne metallic oxides.
A commonly used process for the processing of flue dust is the Waelz process, as disclosed in U.S. Pat. No. 4,525,208, which is also herein incorporated by reference in its entirety. The Waelz kiln process involves mixing carbon, usually in the form of coke or coal, with the flue dust to form a mixture. The mixture is heated in a reducing atmosphere to volatilize the metallic zinc, and the metallic zinc is reoxidized to form zinc oxide, a stable material having a high melting temperature. The process is carried out in a counterflow type rotary kiln. The iron is discharged and the zinc oxide is carried gas-borne along with the kiln exhaust stream. A substantial portion of the heat, if not all of the heat required to initially vaporize the zinc is generated by the burning of the carbon in the carbon/flue dust mixture within the kiln. The combustion process within the kiln requires a substantial amount of air/oxygen, and the flow of this air, and the combustion process itself, generate a significant amount of turbulence within the kiln. As a result of this turbulence, a substantial amount of iron, calcium, silicon and aluminum compounds are also made gas-borne within the kiln, and these contaminate the exhaust stream of potentially valuable zinc oxides.
U.S. Pat. No. 5,013,532, discloses a process using a stream of hydrogen gas to reduce the zinc contained in electric arc furnace dust, followed by volatilizing the metallic zinc, and reoxidizing the metallic zinc to form zinc oxide. A process for removing zinc and other heavy metals from compacted furnace dust and carbon mixture is disclosed in U.S. Pat. No. 5,186,741, which is herein incorporated by reference in its entirety. A process for removing zinc, lead and cadmium from electric arc furnace dust, and metallizing the iron present in the dust to return it to the steel making operation is disclosed in U.S. Pat. No. 5,601,631, which is herein incorporated by reference in its entirety.
In addition to the methods for handling flue dust disclosed above, another known method for processing flue dust involves a rotary kiln that is heated exclusively by a heat source external of the kiln, such as by gas combustion on the outside of the kiln. The carbon necessary for the endothermic chemical reaction (to reduce the zinc and cadmium to a metallic, vaporized form) is supplied from the coke mixed with the flue dust. However, the lack of oxygen within the kiln substantially prevents combustion of the carbon, and essentially no heat is supplied to the flue dust internally of the kiln. Such an indirectly heated rotary kiln process is disclosed in a paper entitled xe2x80x9cFirst Experiences and Results of the BSN-Process to Recover Zinc and Lead from EAF Dustxe2x80x9d, by Karl Haase, published May 17, 1999.
It would be advantageous if there could be developed flue dust handling processes that are even more efficient than those described above. Preferably, such a method would reduce the energy consumed in the process, and would result in greater purity of the separated constituents in the various output streams. Also, ideally, the ability to treat environmentally unfriendly substances such as dioxins and furans would be enhanced.
The above objects as well as other objects not specifically enumerated are achieved by a method of processing flue dust that contains one or more compounds from a first group of zinc, lead and cadmium compounds, and contains iron compounds. The method involves heating the flue dust to cause a substantial portion of one or more of the compounds of the first group to become gas-borne. A carbonaceous material is introduced to the remaining flue dust, and the flue dust/carbonaceous material mixture is heated to cause a substantial portion of the remaining compounds from the first group to become gas-borne while retaining a substantial portion of the iron in a non-gas-borne condition. The gas-borne compounds are separated from the non-gas-borne compounds.
According to this invention, there is also provided a method of processing flue dust that contains lead compounds and iron compounds. The method includes heating the flue dust to cause a substantial portion the lead to become gas-borne, and introducing a carbonaceous material to the remaining flue dust. The flue dust/carbonaceous material mixture is heated to cause additional constituents of the flue dust to become gas-borne while retaining a substantial portion of the iron in a non-gas-borne condition. The gas-borne compounds are separated from the non-gas-borne compounds.
According to this invention, there is also provided a method of processing flue dust that contains one or more compounds from a first group of zinc, lead and cadmium compounds, and contains one or more compounds from a second group of iron, silicon, calcium, magnesium and aluminum compounds. The method includes heating the flue dust to cause a substantial portion of one or more of the compounds of the first group to become gas-borne. A carbonaceous material is introduced to the remaining flue dust, and the flue dust/carbonaceous material mixture is heated to cause a substantial portion of the remaining compounds from the first group to become gas-borne while retaining a substantial portion of the second group in a non-gas-borne condition. The gas-borne compounds are separated from the non-gas-borne compounds.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.